make: Archive Update
11.2 Implicit Rule for Archive Member Targets
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Recall that a target that looks like 'A(M)' stands for the member named
M in the archive file A.
When 'make' looks for an implicit rule for such a target, as a
special feature it considers implicit rules that match '(M)', as well as
those that match the actual target 'A(M)'.
This causes one special rule whose target is '(%)' to match. This
rule updates the target 'A(M)' by copying the file M into the archive.
For example, it will update the archive member target 'foo.a(bar.o)' by
copying the _file_ 'bar.o' into the archive 'foo.a' as a _member_ named
'bar.o'.
When this rule is chained with others, the result is very powerful.
Thus, 'make "foo.a(bar.o)"' (the quotes are needed to protect the '('
and ')' from being interpreted specially by the shell) in the presence
of a file 'bar.c' is enough to cause the following recipe to be run,
even without a makefile:
cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o
Here 'make' has envisioned the file 'bar.o' as an intermediate file.
Chains of Implicit Rules Chained Rules.
Implicit rules such as this one are written using the automatic
variable '$%'. Automatic Variables.
An archive member name in an archive cannot contain a directory name,
but it may be useful in a makefile to pretend that it does. If you
write an archive member target 'foo.a(dir/file.o)', 'make' will perform
automatic updating with this recipe:
ar r foo.a dir/file.o
which has the effect of copying the file 'dir/file.o' into a member
named 'file.o'. In connection with such usage, the automatic variables
'%D' and '%F' may be useful.
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